A CFD approach to the atmospheric dispersion of radionuclides in the vicinity of NPPs

Most studies of atmospheric dispersion of radionuclides released from Nuclear Power Plants (NPPs) are based on Gaussian plume models or on the use of a convection–diffusion equation. Such methods, which do not involve solving the flow problem, are useful in the atmospheric mesoscale, of the order of 2–2000 km from the NPP. However, they do not account for the turbulence generated by the interaction of the wind with obstacles and with the released material stream, which are the dominant factors in the local scale, of the order of 0–2 km from the source of emission. Here, the authors advocate the use of computational fluid dynamics (CFD) to study the dispersion problem. The physical model comprises the Navier–Stokes equations, a convection–diffusion energy equation, and transport equations for the radionuclides. The paper details the stabilized finite element formulation used, stressing its connection with the variational multiscale/large eddy simulation approach. Adaptive techniques combining error estimation and remeshing are also employed. The method is implemented on a Beowulf parallel computing system using domain decomposition and the message passing interface (MPI). Controlled emissions from a chimney and release from severe accidents have been simulated, showing the importance of the local phenomena on the dispersion problem.